In control, detection and navigation systems of various moving objects, not only information of displacement, angular displacement and velocity but also information of acceleration and angular rate are needed. Inertial sensors (including acceleration sensors and angular velocity sensors) are devices for measuring acceleration and angular rate.
Since the end of 1980s, various miniature sensors have been produced with the development of the Micro-Electro-Mechanical-System (MEMS) technique. Due to that MEMS inertial sensor based on MEMS technology can be produced in batch, the disadvantages of the previous inertial sensors such as large size and high cost, are overcome, and thus MEMS sensor has become a dominating trend for many future applications.
The existing MEMS inertial sensor is usually implemented as a capacitive inertial sensor, which generally includes: a fixed electrode for detecting motion of an object; a movable sensitive element (usually referred to as a movable electrode), where the capacitance between the movable sensitive element and the fixed electrode is changed due to the motion of the object; and an electrical signal processor which is electrically connected with the fixed electrode and the movable electrode. In the MEMS inertial sensor, the movable sensitive element usually serves as part of a mass block to reduce the size and the weight of the whole device, and for the mass block, the greater is the mass, so is the inertia.
In the prior art, companies such as Bosch, ST, Freescale and ADI usually use deposited polycrystalline silicon as structural material for fabricating the MEMS inertial sensor (referred to as a polycrystalline silicon method below). The polycrystalline silicon method has the advantage of simple process, but the material has relatively high stress, which affects device reproducibility and increase in film thickness, and therefore limits the size and sensitivity of the inertial sensor. Moreover, due to the poor reproducibility, production yield is reduced, and cost is increased.
U.S. Pat. No. 6,170,332B1 discloses a micromechanical acceleration sensor, which is fabricated by using a single silicon wafer, and each part of the MEMS inertial sensor is formed on the one wafer by etching. But due to the inherent disadvantage of etching technology, i.e., non-uniformity, the performance of the formed sensor such as the reliability may be affected.
Moreover, in the aforementioned US patent, the Z-axis sensor detects the Z-axis acceleration by detecting the change in overlapping area between the capacitor electrodes due to the displacement in the vertical direction, thereby limiting the design of the vertical-direction (Z-axis) sensor.